The handling of very hot concentrated sulfuric acid is a very specialized problem. For example alloys having the very best resistance to hot concentrated sulfuric acid do not perform nearly as well in other corrosive solutions as do other alloys. Conversely the best performing alloys for other applications seldom if ever equal the performance in hot concentrated sulfuric acid of the best alloys designed for the latter purpose.
An iron-base alloy containing 14.5% silicon has been available for many decades for service in hot concentrated sulfuric acid. However, it is non-machinable and at least as brittle as glass. Its great susceptibility to both mechanical and thermal shock severely limits its usefulness. All other alloys to the present time having excellent corrosion resistance to sulfuric acid have been nickel-based or at least of substantial nickel content.
An alloy known as Hastelloy D and consisting of 9% silion and 3% copper in nickel is not as brittle as the silicon-iron alloy but performs well in hot concentrated acid and is, of course, a nickel-based alloy. A variation of Hastelloy D consisting of 9.5% silicon, 2.5% copper and about 3% each of titanium and molybdenum and the balance nickel, has also been developed.
Ordinary austenitic stainless steels of the 18% Cr-8% Ni type can be used in hot sulfuric acid of greater than 98.5% acid strength, but failure is rapid at lower acid concentrations.
All other known alloys developed for service in hot concentrated sulfuric acid have contained large amounts of nickel and chromium with 2.5% to 7% silicon and various amounts of molybdenum and copper. Parr, U.S. Pat. No. 1,115,239, discloses alloys of lower silicon content but they do not possess good resistance to hot concentrated sulfuric acid. However, Parr's alloys were later modified by increasing the silicon content to gain some improvement in performance in hot concentrated sulfuric acid.
Johnson et al, U.S. Pat. No. 2,938,786, and Boyd, et al, U.S. Pat. No. 2,938,787, disclose nickel-based alloys of 22.5% to 28% chromium, 8.5% molybdenum, 5.5% copper, and 2.5% to 7% silicon. These alloys give excellent performance, but are very costly not only because of their high contents of nickel and molybdenum, but also because they require high purity chromium metal for their formulation. Also, they are quite brittle and difficult to machine.
Johnson, U.S. Pat. No. 3,758,296, discloses an alloy of somewhat increased chromium content with reduced nickel content. However, this alloy contains about 6% of the scarce and costly metal cobalt. This alloy is also quite brittle and of very poor machinability.
In my application Ser. No. 234,815 there is disclosed a casting alloy of about 38% Ni, 32% Cr, 4.5% Mo, 3% Cu, 3.6% Si, and 0.55% N. This alloy is less brittle and has better machinability than the alloys of Johnson for hot concentrated sulfuric acid service. It has useful resistance to 85% sulfuric acid to 100.degree. C., to 93% acid to 90.degree. C. and outstanding resistance to 96% to 100% acid solutions to about 110.degree. C.
My application Ser. No. 343,790 discloses an alloy of about 20% Ni, 24% Cr, 0.35% Mo, 3.5% Cu, 3.6% Mn, 2.5% Si, and 0.6% Cb which can be furnished in all wrought forms as well as castings. This alloy has useful resistance to 85% acid to 100.degree. C., to 93% acid to 80.degree. C., to 96% acid to 110.degree. C. and to 98% acid to 200.degree. C. It is a unique alloy in that it has resistance to these acid strengths and yet is capable of being furnished in wrought forms. It is also a low cost alloy. Because of this combination of properties that alloy fills a very important need for manufacturing sulfuric acid resistant pipes, plates, etc, as well as castings.
However, in addition to the corrosion problems associated with handling hot concentrated sulfuric acid, concentrated sulfuric acid streams may contain air bubbles and/or solid particulate matter, both of which tend to cause erosion in cast pump parts and fittings. Thus, there has remained a need for hardenable casting alloys capable of resisting erosion and cavitation while handling very hot concentrated sulfuric acid solutions. There also remains a need for such alloys to be of even lower strategic element content and lower cost than prior alloys.